The present invention relates to an atomizer spray plate of a fuel oil atomizer for pressure-type atomization systems, including spill return systems, and simplex, or xe2x80x9conce-throughxe2x80x9d systems.
For environmental and economical reasons, there is an ongoing need to improve the efficiency of fuel oil atomizers which supply fuel oil to a furnace. It is known that the formation of oxides of nitrogen (NOx) can be slowed by providing fuel-rich and fuel-lean zones in the atomizing spray pattern. Such a fuel spray pattern can be achieved by imparting a rotational momentum, or swirl, to the fuel as it exits the atomizer, and by shaping the fuel spray in a specific manner.
For example, U.S. Pat. No. 5,622,489 to Monro discloses a fuel atomizer with an oblong discharge slot that is shaped to achieve a spray pattern with fuel-rich zones that are spaced apart from one another and separated by a central fuel-lean zone. However, the shaping of the oblong slot is rather complex as the width and angle of the walls of the slot must be precisely set.
Commonly owned U.S. Pat. No. 6,024,301 to Hurley (the xe2x80x9cHurley patentxe2x80x9d) provides an improvement over the design of U.S. Pat. No. 5,622,489 to Monro. The Hurley patent provides a low NOx fuel oil atomizer with an atomizer spray plate having an oblong transverse discharge slot that provides a spray pattern with fuel-rich and fuel-lean zones, yet does not require complex machining of the discharge slot. The Hurley patent also provides a method for fabricating such an atomizer spray plate. Furthermore, the fuel oil atomizer of the Hurley patent is compatible with pressure-type atomization systems, including spill return systems and simplex systems. While the atomizer of the Hurley patent provides improvements over prior art atomizers, the transverse discharge slot results in a flame length which may be too long for use in some restrictive furnace designs.
It would be advantageous to improve upon the atomizer design provided by the commonly owned Hurley patent. It would be further advantageous if such a design provides for similar or improved reductions in NOx emissions, while providing flexibility for a variety of applications. It would be further advantageous to provide an atomizer design having shorter flame lengths for use in applications where the furnace geometry is restrictive.
The present invention provides apparatus and methods having the above and other advantages.
The present invention relates to an atomizer spray plate of a fuel oil atomizer for pressure-type atomization systems, including spill return systems, and simplex, or xe2x80x9conce-throughxe2x80x9d systems.
An atomizer spray plate for discharging fuel oil in accordance with the present invention includes a generally cylindrical rear portion and a generally conical front portion. A frusto-conical whirl chamber extends from the rear portion to the front portion with a decreasing radius. A central longitudinal axis extends through the whirl chamber. Preferably, the rear portion includes a number of whirl slots extending radially inward from an outboard region of the rear portion to the whirl chamber. The whirl slots receive fuel oil at the outboard region and supply the fuel oil to the whirl chamber with a rotational energy.
A discharge slot is provided in the front portion of the atomizer spray plate for receiving the fuel oil from the whirl chamber with the rotational energy.
In particular, the discharge slot includes a cylindrical through-hole with a diameter d. A central longitudinal axis of the through-hole is co-linear with the central longitudinal axis of the whirl chamber. That is, the through-hole is aligned with the whirl chamber.
The discharge slot also includes at least three lobes (i.e. slots) equally spaced about the through-hole and oriented in a radial direction, each lobe having a semi-circular cross-section with radius r. The lobes extend approximately perpendicular to the central longitudinal axis of the cylindrical through-hole.
Advantageously, the discharge slot can be easily and economically fabricated with two shaping steps. Furthermore, there is no need to precisely set any particular non-right angle for walls of the discharge slot. Yet, the discharge slot provides a spray pattern with lateral fuel-rich zones separated by a central fuel-lean zone. This spray pattern has been demonstrated by testing to reduce the peak combustion flame temperature, thereby inhibiting the formation of harmful NOx combustion byproducts.
The front portion of the atomizer spray plate preferably has a generally conical front surface surrounding the discharge slot and sloping at a particular angle, for example between 75 and 85 degrees, relative to the central longitudinal axis of the cylindrical through-hole.
Furthermore, the radius r is selected to be slightly greater than d/2. The lobes are provided at a depth in the front portion to form a desired primary spray angle xcex1 that is defined by a tangent line to the lobes at a forward-most point of the front portion of the spray plate. A secondary spray angle is achieved along a length-wise direction of each lobe.
Preferably, the depth of the lobes is approximately r(1xe2x88x92sin(xcex1/2)), the desired primary spray angle xcex1 is approximately 20xc2x0 to 40xc2x0, and r=d/(2*cos(xcex1/2)).
In a particular embodiment of the invention, three lobes are equally spaced about the through-hole and oriented in the radial direction. In such an embodiment, a developed secondary spray angle of approximately 35xc2x0 to 45xc2x0 may be achieved along a length-wise direction of each of the three lobes.
In an alternate embodiment, four lobes are provided, which are equally spaced about the through-hole and oriented in a radial direction to form two pairs of diametrically opposed lobes. In a four lobe embodiment, a developed secondary spray angle of approximately 70xc2x0-90xc2x0 may be achieved along a length-wise direction of each pair of lobes.
Optionally, a portion of the fuel oil in the whirl chamber is returned to a fuel oil supply instead of being supplied to the discharge slot.
Preferably, a ratio xe2x80x9cAxe2x80x9d/(d*D2) is in a range from approximately 0.4 to approximately 0.6, xe2x80x9cAxe2x80x9d is a total flow area of the whirl slots, and D2 is a diameter of the whirl chamber at a point where the fuel oil is supplied to the whirl chamber from the whirl slots.
Furthermore, a method is presented for fabricating an atomizer spray plate for discharging fuel oil. The method includes the steps of: providing an atomizer spray plate having a rear portion and a front portion, providing a whirl chamber extending from the rear portion to the front portion, where the whirl chamber has a central longitudinal axis extending therethrough, and providing a discharge slot in the front portion for receiving fuel oil from the whirl chamber.
The discharge slot is obtained by providing (a) a cylindrical through-hole with a diameter d having a central longitudinal axis that is co-linear with the central longitudinal axis of the whirl chamber, and (b) at least three lobes equally spaced about the through-hole and oriented in a radial direction, each lobe having a semi-circular cross-section with radius r and extending approximately perpendicular to the central longitudinal axis of the cylindrical through-hole.
The rear portion of the atomizer spray plate is provided with a plurality of whirl slots extending radially inward from an outboard region of the rear portion to the whirl chamber to receive fuel oil and provide it to the whirl chamber with a rotational energy. The fuel oil is then provided to the discharge slot via the whirl chamber.
Those skilled in the art should appreciate that the particular dimensions of the atomizer provided herein are exemplary only. The dimensions and spray angles may be dependent on the furnace application (e.g., constraints of the furnace geometry) and/or the results desired, for example, there may be tradeoffs between NOx emissions, flame length requirements, fuel efficiency, and the like. These variables may be controlled by varying the number of lobes, the spray angles, and other atomizer dimensions. For example, the transverse slot of the Hurley patent may be viewed as a single pair of two diametrically opposed lobes. A three lobe embodiment of the present invention will provide a shorter flame length as compared with the two lobe design of the Hurley patent. Similarly, a four lobe embodiment of the present invention (e.g., two pairs of diametrically opposed lobes) will provide an even shorter flame length than that provided by the three lobe embodiment.